1. Field of the Invention
This invention relates to an ultrasonic inspection system for use in non-destructive testing of titanium and other materials in cylindrical billet form, in which a plurality of transducers are employed to take readings at varying depths through the thickness of the billet.
2. Description of Related Art
In order to assure that only the highest quality titanium is die-forged into aircraft engine disks, the material must be inspected in the billet stage to the best possible sensitivity. Because the billets are hot worked, most of the porosity and shrink present in the cast ingots have been closed and healed. While some porosity can survive the billet process, these pores will probably be healed by the die forging of the disk blanks. Therefore the flaws of primary concern are inclusions, one type of which is a nitrogen stabilized "hard alpha" phase found in titanium. Other undesirable inclusions are oxides and silicates. Other types of flaws, such as voids, are also of concern.
Ultrasonic inspection is employed to attempt to detect all flaws in the materials which would be detrimental to the performance of the material as an aircraft engine spool or disk, especially the aforenoted "hard alpha" inclusions. One difficulty previously experienced in using ultrasonic testing has been in obtaining reliable readings through the entire thickness of the material under test, and in pinpointing the location of the flaws detected.
In the beginning stages of the overall process of die forging titanium aircraft disks, a titanium billet is formed from a cropped cylindrical ingot usually about 30-36 inches in diameter and 7000-10,000 pounds in weight. The ingot is rotary forged into a series of cylindrical billets. These can vary from 6 to 15 inches in diameter. Billets that are six inches in diameter will weigh 55 pounds per foot of length, a billet with a 12-inch diameter will weigh 218 pounds per foot, and 15" diameter billets will weigh 340 pounds per foot. The individual billet segments are seldom less than 10 feet long, and seldom exceed 20 feet in length.
When rotary scanners are used to scan the billets, the maximum inspection depth is equal to the billet radius, or from 3.0" to 6.5" for a 6-13" range in billet diameter. In order to produce uniform test sensitivities, current ultrasonic inspection systems use distance compensated gain (DCG) amplifiers that increase gain as the distance from the transducer increases. This strategy ignores the decrease in acoustic signal to noise (S/N) ratio produced by beam spread. Because acoustic noise is also amplified with system gain, and titanium has high acoustic noise, DCG combined with beam spread results in low flaw S/N ratios, particularly at the billet centerlines. Since the primary factor limiting detection is acoustic noise, the use of DCG techniques produces no improvement in flaw S/N or in the probability of detection (POD) for flaws.
Current ultrasonic systems scan billets with small (1"") cylindrically focused transducers. These small sizes produce diffraction limited near field limits (foci) at depths of less than 2.0" in the billets rather than near the billet centerline. Therefore, all of the billet sizes insonified with these transducers have diverging beams at the centerline, which is the most likely region in which hard alpha flaws are present. Larger transducers have been used by some European manufacturers, such as the French titanium supplier CESUS, but three or fewer such transducers are used in the inspection process, and careful attention has not been given to the uniformity of the beam overlap.
The current ultrasonic inspection systems (including those used by CESUS) do not produce uniform ultrasonic interrogation of the billet interior. Quasi-uniform test sensitivity is produced by DCG amplifiers. This does amplify the weaker signals resulting from deeper reflectors and therefore permits the use of a single amplitude threshold at all depths. However, as discussed above, DCG amplifiers operate on both the flaw signal and the noise and therefore do not improve flaw detection.
Recently, several phased array systems have been evaluated for billet testing. The phased array generally involves transmitting a large number of ultrasonic pulses into the material, with the numerous pulses focused at different depths. The most serious limitation in the use of phased array transducer systems for testing large material volumes is the high pulse repetition rates required for the single phased array transmitter to transmit the multiple focused beams at the number of billet depths necessary to obtain the desired flaw detection capabilities. In thick material sections, high repetition rates always result in unattenuated sound and these remnant pulses result in a high number of false flaw detections. In order to reduce the repetition rate, low scanning rates must be used, resulting in long inspection times. The trade-offs between long inspection times, high false calls and the current high cost of multiple channel phased array systems make phased arrays impractical for titanium billet testing.
In an initial attempt to overcome these disadvantages, a multiple transducer inspection system was developed in which a linear array of transducers focused at increasing, and overlapping, depths of focus was employed to insonify the billet as the billet was rotated in the immersion tank. The output of that system was in the form of a strip chart on which perturbations in the line tracings of the output of each transducer channel were used to locate potential flaws in the billet.
While representing a distinct improvement over then-existing inspection methods, this inspection method also evidenced several significant limitations. As an example, the strip chart provides no direct spatial correlation of the output to the billet itself. In order to partially overcome this limitation, the inspection apparatus was configured such that the strip chart output would be identical in length, i.e., up to 20 feet, to the billet being inspected. This enabled a physical comparison of the strip chart output to the billet, by overlapping the strip chart on the billet, wherein perturbations in the output could be traced at least to a specific axial location on the billet.
The strip chart output also was purely analog in form, as the chart was the only recording and/or storage medium, and no electronic storage of the output data was accomplished. Thus, there was no ability to view the results in any different form which might be of greater assistance in increasing the probability of flaw detection and in lowering the number of false flaw indications. The dynamic range of the strip chart output was also limited, and thus further limited the ability to accurately detect and pinpoint the location of flaws. In addition, it became apparent that the retention and archiving of the strip charts, which was the only form in which the data was recorded, would be extremely cumbersome.